Fly-by-Wire: Redefining flight and maintenance

Fly-by-Wire Redefining flight and maintenance By Jim Sparks September 2001 One of my favorite toys as a youngster was a model aircraft equipped with a single-cylinder, air-cooled engine fueled with cigarette lighter fluid. The method of...


Fly-by-Wire

Redefining flight and maintenance

By Jim Sparks

September 2001

One of my favorite toys as a youngster was a model aircraft equipped with a single-cylinder, air-cooled engine fueled with cigarette lighter fluid. The method of control was a simple handgrip. The aircraft was tethered to the grip with two wires (actually, heavy fishing line) one of which insured the toy would always circle the holder, while the second wire was linked to a movable horizontal stabilizer. The instruction book provided directions for successful operation involving a whole series of maneuvers including figure eight’s and landings. I was usually able to get the machine in flight, but my landing techniques were still far from those described in the directions. This was my first exposure to "fly-by-wire" technology.

Evolution of flight control systems
The evolution of flight control systems is the one primary factor that has brought us forward from the days of the Wright Brothers. As aircraft became larger and engine power increased, more force was required to manipulate the flight control systems. Soon control surfaces were assisted by aerodynamic tabs, pneumatic cylinders and hydraulic actuators. These conventional systems can often be a limitation in the initial design phase of a new aircraft and will affect wing size and shape along with location and size of stabilizers. With the onset of the age of electronics, it was only natural to anticipate the removal of push-pull rods and cables as the primary means of supplying commands to flight controls and replacing them with wires. Now the pilot’s actions are interpreted by an electrical signal and then the order is carried out by an actuating device. A fly-by-wire system would be lighter, and more responsive, to pilot input. In addition, military aircraft would be less vulnerable to battle damage. The end result is an increase in efficiency, safety and performance. Vulnerability of aircraft using long cable runs has unfortunately been demonstrated on several commercial airliners. Once in the mid-1970s, a main entry door blew off in flight and the resulting collapse of the floor caused loss of use of the control cables. What happened next requires no explanation. With the redundancy demanded in the certification of transport aircraft, the routing of wires to provide electronic flight control operations would be such that any one failure should not be able to disrupt safe operation of the aircraft. In addition to improved safety and handling, significant weight savings can be expected by using wires rather than cumbersome and complex cable or link rod systems.

Redefining flight and maintenance
Over the years, "fly-by-wire" has taken on a whole new meaning. Both Boeing and Airbus are utilizing computer technology on current production aircraft and Dassault announced its next business jet will also be fly-by-wire. Enhanced aircraft responsiveness and reduced pilot workload proven by supersonic military aircraft will no doubt make electronics the control method of choice. In fact, studies are under way to adapt these principles to general aviation aircraft. Just imagine working an inspection where flight control cables don’t require lubrication, replacement or even a visual check. In fact, the majority of the system can be completely tested by sitting in the flight deck and electronically interrogating a whole series of flight computers using onboard maintenance computer diagnostics. Flight control trim tabs will no longer be needed. External mechanical aerodynamic enhancements will be replaced with an optimized control surface that is specifically placed by a computer in a fraction of the time it would take a man to react. Airbus has already eliminated the stick and yoke and has gone to side mounted "joysticks" for both pilot and co-pilot. Boeing, on the other hand, still uses a conventional means of flight control input.
Although the concept may sound simple, the engineering challenges are immense. First of all, software has to be written and tested so that bug-free operation can be expected. This, in itself, takes tremendous hours of programming. Hardware also has to be developed that will implement the orders of the software using pinpoint timing and accuracy.

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